Two basic approaches are currently being employed in the design of monolithic low noise amplifiers. The conventional approach uses an input FET in a common-source configuration with the necessary input matching circuitry to achieve minimum noise figure. See Lehmann, Brehm, and Westphal, "10 GHz 3-Stage Monolithic 4 dB Noise Figure Amplifier," 1982 IEEE International Solid-State Circuits Conference Digest of Papers, Feb. 11, 1982, pp. 140-141 which is hereby incorporated by reference. This design can achieve very low noise figure, but also has a high input voltage standing wave ratio (VSWR) (typically 3:1 or 4:1) because the input matching circuit presents the required noise match impedance for minimum noise figure rather than a conjugate match. Because of the high input VSWR, this design must incorporate a 3 dB hybrid (coupler) at the input and output to integrate into any receiver. This balanced configuration requires a matched pair of amplifiers as well as 3 dB hybrids at the input and output. This approach, when implemented monolithically, consumes a great deal of GaAs real estate.
The second approach employs an input FET in a common-gate configuration, often referred to as "active-matching". This design achieves good input VSWR but at the expense of degrading the noise figure. Such an approach has been demonstrated by Petersen et al, see "Monolithic GaAs Microwave Analog Integrated Circuits", Interim Report for ERADCOM, September 1980, which is hereby incorporated by reference; and Estreich, "A Wideband Monolithic GaAs IC Amplifier," 1982, IEEE International Solid State Circuits Conference Digest of Papers, Feb. 11, 1982, pp. 194-195 which is hereby incorporated by reference. See also Pengelly et al, "A Comparison between Actively and Passively Matched S-band GaAs Monolithic FET Amplifiers", MTT Symposium Digest of Papers 1981, pp. 367-369, which is hereby incorporated by reference. In this prior art, the common-gate FET is operated at a bias condition to achieve a device transconductance, g.sub.m, of approximately 20 mS. Because the input impedance of a common-gate FET is approximately 1/g.sub.m, the input impedance is nominally 50 ohms. At this bias condition, input VSWR is optimized over a broadband of frequency, but one sacrifices often the most critical performance parameter, noise figure. Computer modeling at TI has shown that noise figure can be considerably improved if the FET can be operated at higher values of transconductance.
In the prior art, balanced amplifier techniques are commonly used to achieve good input and output match. However, such balanced techniques require that the entire amplifier be duplicated, and thus gross economies of semiconductor real estate are required. It would be highly desirable to have a single-ended amplifier design, i.e. an amplifier design which did not require duplication of active stages, which also provided good noise figure and input matching.
Thus, it is an object of the present invention to provide a single-ended low-noise amplifier with good input matching.
A great many high-frequency system applications require a low-noise input stage which has good input impedance matching at microwave frequencies.
A principal area of developing technology, where a crucial need for low-noise amplifiers exist, is in the area of microwave receivers. The crucial first step in any microwave receiver system is to amplify the rf signal, as received at the antenna, to raise it above the background of thermal and other noise. It is particularly desirable to be able to provide such a low-noise input stage in an integrated embodiment. Such low-noise input stages are required in, for example, conventional radar, phased-array radar, uplink and downlink stages of satillite communications receivers, home TV RO (television receive only) systems, police radar, intrusion alarm systems, microwave point-to-point relay systems, and mobile radio systems.
A further area of applications, where critical need for such low-noise integrated input stages exists, is in sensors. In many sensor applications, the compactness and economy permitted by an integrated design is even more critical than in receivers. In particular, low-noise microwave amplifiers are needed for pollution monitoring equipment. In such systems, microwave resonant absorptions characteristic of various pollution components can be used to determine the type and density of a particular pollutant with extreme precision, by remote microwave sensing. Similarly, in petrochemical processing, microwave sensing can be used for very accurate detection of the percentage of methane or other process components in a process gas stream. Low-noise microwave amplifiers are also needed for sensing of environmental microwave radiation, to detect and measure possible human health hazards. Finally, an additional biomedical application is radiometry, in which the amount of microwave radiation applied to or transmitted through living tissue must be precisely measured.
Thus it is an object of the present invention to provide a low-noise microwave amplifier having good input impedance match.
It is a further object of the present invention to provide an integrated low-noise microwave amplifier having very good impedance match.
It is a further object of the present invention to provide a microwave receiver systems having very low system noise temperatures.
It is a further object of the present invention to provide microwave receiver systems having very low system noise temperatures in room temperature operation.